The present invention relates generally to catalytic combustors for gas turbine power plants and, more particularly, to support structures or holders for catalyst beds, especially modular catalyst beds.
As a means for meeting increasingly strict environmental codes, catalytically-supported thermal combustion systems for gas turbines are being developed. Some catalytic combustors have the potential to provide a low emission, energy-saving, and high combustion-efficiency system. Also, a catalytically-supported thermal combustion system is of relatively small size as compared to an exhaust clean-up system, for example employing catalytic DeNOx in the exhaust. An exhaust clean-up system must process approximately fifteen times the volume of gas compared to a catalytic combustor due to the relatively low pressure and high temperature of the exhaust.
The catalyst bed is preferably a monolithic or unitary structure comprising a carrier of high temperature relatively fragile ceramic material formed into a honeycomb-like structure comprising a multiplicity of thin-walled axially-extending channels. The actual catalytically active material is either carried on the surface of the ceramic substrate, or impregnated therein, and may be a noble metal or a base metal oxide of such elements as zirconium, vanadium, chromium, manganese, copper, platinum, palladium, iridium, rhodium, ruthenium, cerium, cobalt, nickel, iron, and the like.
By way of example, catalytic combustors for gas turbines are disclosed in the Pfefferle U.S. Pat. Nos. 3,928,961 and 4,019,316, and in the DeCorso et al. U.S. Pat. No. 3,943,705. The two Pfefferle patent disclosures in particular discuss the distinction between "catalytically-supported thermal combustion", which involves high temperature and high reaction rate thermal combustion in virtually the entire gas stream within the catalyst bed, and conventional catalytic combustion, which involves reactions taking place at the surface of the catalyst.
Up until the present time, most development work on catalytic combustors for gas turbines has been on bench scale systems with the objective of demonstrating the low emissions characteristics of these combustors with various fuels and over a range of operating conditions. However, few proposals have been directed to the problems of providing long term mechanical integrity of a catalytic combustor, or to the means for starting up and controlling the operation of a complete combustion system over the fuel-air ratio range needed for a practical gas turbine engine. Systems or parts of systems described in the literature either ignore one or more of these problems, or are mechanically very complex due to variable geometry or other such means for limiting the fuel-air ratio entering the catalyst bed.
A system which does address these concerns is disclosed in commonly-assigned U.S. patent application Ser. No. 290,568, filed Aug. 6, 1981, by Davis, Jr. et al. entitled "MODULAR CATALYTIC COMBUSTION BED," now abandoned. That application discloses the overall concept that, as a matter of mechanical integrity, it is highly preferable to separate the catalyst bed into a plurality of individual catalyst modules. As one result, potentially destructive forces resulting from thermal gradients are minimized.
The present invention is directed to more specific aspects of the mechanical design of catalytic combustors, specifically to suitable holders for catalyst beds operating in the environment of a gas turbine combustor. Preferably, as disclosed in the above-identified application Ser. No. 290,568, the catalyst bed is divided into a plurality of modules each comprising a catalyst bed and support structure; in accordance with the present invention, these modules are integrated into a full scale combustor capable of meeting the requirements of a modern gas turbine combustion system. However, the support structure concepts of the present invention need not be limited to modular combustors; they may be applied to combustors having single catalyst beds as well.
A variety of support structures for catalyst beds have been described in the literature. For example, in the literature reference Hung, W. S. Y., Dickson, W. H., DeCorso, S. M., "Preliminary Design Analysis of a Catalytic Ceramic Structure in a Turbine Combustor", ASME Paper 78 WA/GT-10 (1978), there is disclosed a support structure for a single large catalytic ceramic element, which support structure comprises an air-cooled coiled wire compressed between an outer support cylinder and a layer of castable ceramic on the outer cylindrical surface of the catalyst substrate. Purge and cooling air is metered through apertures in the outer support cylinder into the annular space occupied by the air-cooled coiled wire, and flows upstream to meet and join the main combustor gas flow at the entrance to the catalyst bed. Also, there is an integral cast radially outwardly extending flange at the downstream (hot) end of the catalyst bed, this flange engaging against an element carried by the outer support cylinder.
Another example is disclosed in the literature reference Lew, H. G., Dickson, W. H., DeCorso, S. M., Olson, B. A., Heck, R. M. "Experimentally Determined Catalytic Reactor Behavior and Analysis for Gas Turbine Combustors", ASME Paper 79-GT-150 (1979). The catalyst bed holder disclosed in this particular literature reference employs a solid (non-porous) outer support cylinder, with thermal insulation of a formed blanket or block type filling an annular cavity defined between the outer shell and the actual catalyst bed. An upstream lip on the catalyst bed bears against a complaint metal element carried by the outer support cylinder, and is held in place by pressure drop across the catalyst bed. Purge and cooling air flow is introduced near the downstream end of the annular insulation space, and travels upstream through the insulation to emerge near the front face of the catalyst bed, and be combined with the main flow of gases through the combustor.
Still another form of holder is disclosed in the following two literature references, although not discussed in great detail: Krill, W. V., Kesselring, J. P., Chu, E. K., "Catalytic Combustion for Gas Turbine Applications", ASME Paper 79-GT-188 (1979); and Krill, W. V., Kesselring, J. P., Chu, E. K., Kendall, R. M., "Catalytic Combustion for System Applications", ASME Paper 79-HT-54 (1979). In these literature disclosures, it appears that spoked rings support the catalyst bed at both the upstream and downstream ends, and a large, annular air passageway surrounds the actual catalyst bed. The catalyst bed includes an outer ceramic cylindrical shell.